Most independent craft that orbits the earth contain attitude control, attitude reference, and energy storage systems. Efficient attitude control, attitude reference, and energy storage systems are especially important for satellites, where weight, longevity, and accuracy are crucial considerations. Existing satellites utilize separate subsystems to perform attitude control, provide attitude reference, and store energy. Attitude control is typically provided by reaction or momentum wheels, while attitude reference is provided by inertial sensors and devices that provide an absolute reference such as sun sensors or star trackers. Energy storage is typically performed by batteries.
Satellite attitude can be controlled by applying torques through reaction wheels or control moment gyros. Typically, accuracy is compromised because conventional ball and gas bearings are unable to precisely maintain the orientation of the wheel's angular momentum with respect to the satellite. Inertial sensors used for attitude reference consist of single and two degree of freedom gyros which are also typically supported by ball or gas bearings. Either suspension has a rate life that is shorter than a typical satellite mission, thus requiring redundancy, and therefore added weight, of these systems.
Power is typically supplied to a satellite through photovoltaic arrays. An energy storage system is therefore essential to provide power during the periods when the arrays cannot supply power and to provide additional power for peak loads. Batteries provide electrochemical storage. Older satellite designs incorporate nickel-cadmium batteries, while newer designs use nickel-hydrogen batteries having improved energy density and life. Both battery types, however, are still relatively heavy and gradually lose storage capability over time. Flywheels do not degrade over time and therefore maintain their full energy storage capacity throughout their life. However, their weight must be added to that of additional systems required for attitude reference and attitude control.